The operation of digital transmission systems required certain characteristics in the incoming digital data streams at receivers and regenerators. The proper clock recovery of NRZ (non-return-to-zero) digital signals requires a minimum density of transitions in the digital stream, transition being a change in signal voltage level caused by a change in the signal from binary 1 to binary 0, or vice versa. Also, in accoupled systems, the average density of binary 1's to binary 0's should approximately satisfied through the scrambling of the data signals with a repetitive digital pseudo random sequence which will provide the required transitions and average density on the high speed lines which carry the multiplexed digital signals. At any receiver or regenerator in the system the scrambling can be removed by means of a descrambler to yield the data signals.
Present day fiber optic time division multiplexed systems operate in the multi-gigabit frequency range. Since bandwidth is limited by the electronic devices and not by the fiber itself, it is highly desirable to perform the complex electronic processing at the low speed parallel (or tributary) signal levels at the inputs of multiplexers and the outputs of demultiplexers. In this way the highest-speed circuitry is limited to the final multiplexer stages and to the initial demultiplexer stages, which can be merely bit interleavers and deinterleavers.
Thus it is desirable to perform the aforementioned scrambling at parallel or tributary levels where bit rates are low. In general, the independent (or uncoordinated) scrambling of parallel signal tributaries at the input of a multiplexer does not produce the same high speed line statistics (or high speed scrambling sequence) as compared to performing a single scrambling operation at the serial or high speed level at the multiplexer output. The line conditioning produced by independent tributary scrambling cannot be better than that produced by high speed For example, if independent parallel scrambling codes or sequences each have any given number of consecutive 1's or 0's therein, where N is the number of lines bing multiplexed. For the above reasons, independent tributary scrambling has been only used when N is small, or in combination with simple line coding. In this regard see F. D. Waldhauser, A 2-level, 274 Mb/s Regenerative Repeater for T4/M, Proc. IEEE Int. Conf. on Communications., pp 48.13-17, 1975; J. R. Stauffer, FT 3C- A Lightwave System for Metropolitan and Intercity Applications, IEEE J. Selected Areas on Communications, Vol. SAC-1, pp 413-419, April 1983; and T. Minami, et al, A 200 M bits/s Synchronous TDM Loop Optical LAN Suitable for Multiserver Integration, IEEE J. Selected Areas on Communications, Vol. SAC-3, pp 849-858, 1985.
The present invention describes techniques and circuitry wherein the scrambling of the parallel tributary signals is coordinated in such a way that it produces any desired and predictable high speed line statistics just as though the scrambling were done in the serial high speed line. Previous efforts in tributary scrambling are reported in 0. Brugia, et al, Multiport Modula-2 Generators of Pseudorandom Binary Sequences, Proc. IEEE Int. Symp. on Circuit and Systems, pp 852-855, 1982, in which a d-transform approach is used. That approach is a design procedure using combinatorial circuitry in which some of the feedback register stages are connected to each tributary input line of an N to 1 multiplexer to produce an N times high speed line m-sequence. Besides the difficultly of designing combinatorial circuitry, the multiplexing factor, N, is limited to integral powers of 2.
The present co-inventor Sang Hoon Lee in a copending application entitled Multiplex Digital Communications System, Ser. No. 921,522, filed on Oct. 22, 1986, discloses much simpler techniques and circuitry for utilizing the properties of m-sequences, also known as pseudo random sequences, to achieve coordinated tributary scrambling and hence predictable high speed line conditioning.